OPA4684M Datasheet, PDF (20/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
Figure 50. Low-Power, Differential 4th-Order, 100MHz Butterworth
low-power DSL transceiver interface
With four amplifiers available, the quad OPA4684 can meet the needs for both differential driver and receiver
in a low-power xDSL line interface design. Figure 51 shows a simplified design example. Two amplifiers are
used as a noninverting differential driver while the other two implement the driver echo cancellation and receiver
amplifier function. This example shows a single +12V design where the drive side is taking a 2-Vp-p maximum
input from the transmit filter and providing a differential gain of 7, giving a maximum 14-Vp-p differential output
swing. This is coupled through 50ĆΩ matching resistors and a 1:1 transformer to give a maximum 7 Vp-p on a
100ĆΩ line. This 7 Vp-p corresponds to a 10-dBm line power with a 3.5 crest factor.
The differential receiver is configured as an inverting summing stage where the outputs of the driver are
cancelled prior to appearing at the output of the receive amplifiers. This is done by summing the output voltages
for the drive amplifiers and their attenuated and inverted levels (at the transformer input) into the inverting inputs
of each receiver amplifier. The resistor values are set in Figure 51 to give perfect drive signal cancellation if the
drive signal is attenuated by 1/2 going from the drive amplifier outputs to the transformer input. The signal
received through the transformer has a gain of 1 through the receive amplifiers. Higher gain could easily be
provided by scaling the resistors summing into the inverting inputs of the receiver amplifiers down while keeping
the same ratio between them.
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